The present disclosure concerns a method of calibrating a rotor tip clearance arrangement. It also concerns a rotor tip clearance arrangement calibrated using the method and a gas turbine engine including such a rotor tip clearance arrangement. It also concerns a method of monitoring deterioration of a rotor tip clearance arrangement.
A rotor, for example a turbine or compressor stage in a gas turbine engine, typically includes blades which extend from a hub. Each blade has a tip distal to the hub. An annular rotor casing surrounds the tips of the blades with a small running clearance between the tips and the casing. A rotor is more efficient if the tip clearance is minimised, so that the maximum amount of air approaching the rotor is passed over the aerofoil surfaces of the blades rather than leaking over the tips. However, a small clearance is required to prevent any of the tips rubbing against the casing and thereby eroding either or both components.
During use of the rotor the tip clearance may change, for example through differing rates of thermal and centrifugal growth, damage to or deterioration of the rotor or casing, and accretion of deposits on the rotor or casing. A rotor tip clearance arrangement is typically provided to control the tip clearance during use of the rotor. Such arrangements generally include a tip clearance control actuator.
Known rotor tip clearance arrangements include air flow valves which direct cooling air onto the casing to retard its thermal growth or to cause it to shrink towards the blade tips. Other known rotor tip clearance arrangements include mechanical actuators to move the casing, or segments mounted to the interior thereof, towards or away from the blade tips. Mechanical actuators may be controlled by electrical or electronic actuators.